X-ray reflectivity study of ultrathin liquid films of diphenylsiloxane-dimethylsiloxane copolymers

Using X-ray reflectivity, we observe drastic differences in the interfacial structure and molecular ordering of diphenylsiloxane-dimethylsiloxane copolymer thin films deposited on hydroxylated versus H-terminated (etched) silicon wafers. We find that substrate type and comonomer ratio determine the conformational arrangements in these liquid films. High-energy bonding between the substrate and the molecules and an increase in rigidity of the molecules due to replacement of methyl groups by phenyl groups leads to a specific molecular ordering at the liquid/solid interface and pronounced density oscillations in this region. The observed structural reorganizations are explained by the interplay and the established balance between the chain flexibility and the polymer-substrate interactions.     

Photoresponsive monolayers on water and solid surfaces

Organic photochromic units and molecules can be regarded as light-driven nano molecular machines. Once such molecules are aligned at a surface, the supramolecular organization provides an efficient macroscopic mechanical response in a collective way. Amphiphilic polymers having an azobenzene (Az) side chain are the favorable materials for observation of such effects since they show marked photomechanical response with essentially full reversibility. An in situ Brewster angle microscopic observation showed marked morphological and rheological photoinduced changes in the molecular films. Moreover, we have newly found that the identical photosensitive molecular film transferred on to a solid mica surface shows large morphological changes under highly humid conditions as proven by atomic force microscopy (AFM). It is supposed that the molecular film is driven in the same mechanism both on water and water-adsorbed mica surfaces. These microscopic observations provide new insights of the photomechanical response in photochromic monolayers.     

Effect of Surface Roughness on Structure and Dynamics in Thin Films

We performed Monte Carlo simulations of free‐standing, amorphous polyethylene (PE) thin films at 509 K. The three films are constructed from 9, 36, or 144 independent parent PE chains, with 100 carbon atoms per chain. The two‐dimensional periodic cross‐sectional area of the simulation box is proportional to the number of independent parent chains, with the 144‐chain film having an area four times larger than the 36‐chain film. All three films have a similar bulk density and a comparable thickness between the two free surfaces. The 144‐chain film with the largest periodic surface area has a broader density profile due to the increased roughness of its surfaces. Snapshots of its surfaces along the trajectory indicate dynamic changes in the high and low regions of the rough surfaces. Diffusion of the chains parallel to the free surfaces is suppressed in the 144‐chain film, due to increased surface roughness. The tendency of bonds to orient parallel to the free surface is less pronounced in films with higher surface roughness.     

Effect of nitrobenzene in the styrene–cellulose acetate graft polymerization system

The effect of nitrobenzene, a polymerization retarder, upon the direct γ-ray-induced graft polymerization of styrene onto pyridine-swollen cellulose acetate film was studied. When the films were not highly swollen, small nitrobenzene additions caused an increase in the amount of grafted polystyrene and grafted cellulose acetate. However, when the substrate was highly swollen, nitrobenzene additions reduced the amount of grafted polystyrene without pronounced changes in the amount of grafted cellulose acetate. The number-average molecular weights of the grafted side chains were always two to three times those of the homopolystyrene formed in the bulk monomer solution, which is indicative of hindered chain termination within the substrate film under all reaction conditions. Nitrobenzene additions prevented polystyrene crosslinking reactions, probably because the termination reactions in the presence of nitrobenzene occur by disproportionation rather than by coupling of chain ends. Viscometric results indicated that the polystyrene side chains were branched.     

Preparation and characterization of pure and mixed monolayers of poly(ethylene glycol) brushes chemically adsorbed to silica surfaces

We prepared pure and mixed monolayers of methoxy-terminated poly(ethylene glycol)s (m-PEG's) chemically attached to silica surfaces by using m-PEG silane coupling agents of three different molecular weights. These films were subsequently characterized in water by atomic force microscopy (AFM). Images of pure m-PEG monolayers showed the formation of polymer brushes on silica. Force curves between two modified surfaces suggested that an increase in the number of oxyethylene (OE) groups from 6 (PEG6 surface) to 43 (PEG43 surface) to 113 (PEG113 surface) decreased the flexibility of the m-PEG chains in the m-PEG brushes. Frictional force measurements also showed that the friction increased in the order PEG6 < PEG43 相似文献   

PNIPAM chain collapse depends on the molecular weight and grafting density

This study demonstrates that the thermally induced collapse of end-grafted poly(N-isopropylacrylamide) (PNIPAM) above the lower critical solution temperature (LCST) of 32 degrees C depends on the chain grafting density and molecular weight. The polymer was grafted from the surface of a self-assembled monolayer containing the initiator (BrC(CH3)2COO(CH2)11S)2, using surface-initiated atom transfer radical polymerization. Varying the reaction time and monomer concentration controlled the molecular weight, and diluting the initiator in the monolayer altered the grafting density. Surface force measurements of the polymer films showed that the chain collapse above the LCST decreases with decreasing grafting density and molecular weight. At T > LCST, the advancing water contact angle increases sharply on PNIPAM films of high molecular weight and grafting density, but the change is less pronounced with films of low-molecular-weight chains at lower densities. Below the LCST, the force-distance profiles exhibit nonideal polymer behavior and suggest that the brush architecture comprises dilute outer chains and much denser chains adjacent to the surface.     

Lyophilic properties of surfactant-rich phases of polyethoxylated alkylphenols formed at cloud point temperature

The influence of the concentration conditions, solutions acidity, and electrolyte additions on the lyophilic properties of the surfactant-rich phases of polyethoxylated alkylphenols OP-7 and OP-10 formed at cloud point temperature were studied. The lyophilic properties of surfactant-rich phases were determined by estimating of their effective hydration values and solvation free energy of methylene and carboxyl groups at cloud point extraction of aliphatic monocarboxylic acids. It was shown that the surfactant-rich phases formed from the dilute surfactant solutions have more hydrophobic properties than the phases formed from the high concentrated solutions. The possibility of changing the lyophilic properties of surfactant-rich phases by electrolyte additions was shown: complex formation between electrolyte cation and the polyoxyethylene chain of the surfactant increases the hydrophilic properties of the surfactant-rich phases. Calculations of the solvation free energy of methylene and carboxylic fragments of the aliphatic carboxylic acids at micellar extraction showed the uniqueness of the surfactant-rich phases which are able to energetically advantageously extract both hydrophilic and hydrophobic molecules of substrates.     

Infrared spectroscopic study of stereo-controlled poly(N-isopropylacrylamide) with an extended chain conformation induced by adsorption on a gold surface

Poly(N-isopropylacrylamide) (PNiPAM) compounds with various diad tacticities were prepared, and the molecular interaction properties in a thin film deposited on a gold surface were analyzed using infrared spectroscopy. The intramolecular and intermolecular interactions were found to depend on the tacticity, and only atactic (diad ratio 46 %) PNiPAM exhibits poor molecular interaction even in the bulk sample. On the other hand, the same series of compounds dissolved in an acetone solution were spread on a gold surface to form a thin film. In the dissolution process, the polymer molecules are relaxed via solvation, and they are bound to the gold surface by a molecular interaction to form a submonolayer thin film. In the thin film, the molecular interaction with the gold surface via the N–H group was monitored in the infrared spectra only for a nearly isotactic (m?=?90) PNiPAM by an apparent shift of the N–H stretching vibration band. This shift was confirmed by changing the degree of hydrophilicity of the gold surface: a larger shift is found on a gold surface with stronger hydrophilicity. As a result, the conformation of a nearly isotactic molecule is found to be extended by the interaction with the gold surface, which works to immobilize the molecule.     

Dynamics of dewetting at the nanoscale using molecular dynamics

Large-scale molecular dynamics simulations are used to model the dewetting of solid surfaces by partially wetting thin liquid films. Two levels of solid-liquid interaction are considered that give rise to large equilibrium contact angles. The initial length and thickness of the films are varied over a wide range at the nanoscale. Spontaneous dewetting is initiated by removing a band of molecules either from each end of the film or from its center. As observed experimentally and in previous simulations, the films recede at an initially constant speed, creating a growing rim of liquid with a constant receding dynamic contact angle. Consistent with the current understanding of wetting dynamics, film recession is faster on the more poorly wetted surface to an extent that cannot be explained solely by the increase in the surface tension driving force. In addition, the rates of recession of the thinnest films are found to increase with decreasing film thickness. These new results imply not only that the mobility of the liquid molecules adjacent to the solid increases with decreasing solid-liquid interactions, but also that the mobility adjacent to the free surface of the film is higher than in the bulk, so that the effective viscosity of the film decreases with thickness.     

Molecular hydrogen-induced nucleation of hydrogenated silicon nanocrystals at low temperature

Highly crystallized hydrogenated silicon layers were obtained via the treatment of hydrogenated polymorphous silicon films in a molecular hydrogen ambient. This contrasts other postdeposition studies that obtained nanocrystalline silicon films but necessitated either a plasma activation or high-temperature annealing. The structure of the samples was analyzed by Raman spectroscopy to determine the crystallite volume fraction, which was found to increase up to 80% within 1 hour of treatment. Atomic force microscopy (AFM) showed that the roughness of the surfaces was found to increase after the H₂ treatment. Optical transmission and spectroscopic ellipsometry revealed the pronounced porosity of the films characterized by a static refractive index that is below three, which is a low value for hydrogenated silicon films and a void fraction that is around 15% in the bulk of the films. The effect of the hydrogen molecules on the structure of the films was discussed in terms of the compressive stress exerted by the molecules, trapped in structural inhomogeneities, on the amorphous tissue. It is suggested that for this process to take effect, the films need to be porous and that the amorphous network needs to be in a “relaxed” state.     

Thinning of foam films of micellar surfactant solutions

Drainage in microscopic circular foam films depends significantly on the radial (tangential) mobility of the film surfaces and is accelerated as compared to the limiting case of tangentially immobile surfaces, where velocity of thinning is described by the classical Reynolds’ equation (outflow of viscous fluid from a cylindrical gap between two solid plates). The structure and composition of the adsorption layer and the interfacial mass transfer determine the tangential mobility of the film surfaces and, hence, the measured velocity of film thinning. Experiments with soluble surfactants below the critical micelle concentrations (CMC) have exhibited the effect of dynamic interfacial elasticity. At relatively low bulk concentrations, the interfacial mass transfer is governed by surface diffusion; close to CMC (saturated adsorption layer), the limiting case of tangentially immobile surfaces can be reached and at concentrations above the CMC the film thinning is accelerated again. Here, we report freshly established data on the kinetic behavior of foam films from micellar solutions of soluble nonionic surfactants (decyl-octaoxyethylene alcohol and dodecyl-octaoxyethylene alcohol) in a wide range of concentrations above the CMC aiming to investigate the effect of partially disintegrated micelles acting as sources of surfactant molecules at the surface.     

Local dynamic mechanical properties in model free-standing polymer thin films

High-frequency sinusoidal oscillations of a coarse-grained polymer model are used to calculate the local dynamic mechanical properties (DMPs) of free-standing polymer thin films. The storage modulus G(') and loss modulus G(") are examined as a function of position normal to the free surfaces. It is found that mechanically soft layers arise near the free surfaces of glassy thin films, and that their thickness becomes comparable to the entire film thickness as the temperature approaches the glass transition T(g). As a result, the overall stiffness of glassy thin films decreases with film thickness. It is also shown that two regions coexist in thin films just at the bulk T(g); a melt-like region (G(')G(")) in the middle of the film. Our findings on the existence of a heterogeneous distribution of DMPs in free-standing polymer thin films provide insights into recent experimental measurements of the mechanical properties of glassy polymer thin films.     

Combined atomic force microscopy and fluorescence correlation spectroscopy measurements to study the dynamical structure of interfacial fluids

We have studied the dynamic structure of thin (approximately a few nanometers) liquid films of a nearly spherical, nonpolar molecule tetrakis(2-ethylhexoxy)silane (TEHOS) by using a combination of atomic force microscopy (AFM) and fluorescence correlation spectroscopy (FCS). Ultra-sensitive interferometer-based AFM was used to determine the stiffness (force gradient) and the damping coefficient of the liquid film. The experiments show oscillations in the damping coefficient with a period of approximately 1 nm, which is consistent with the molecular dimension of TEHOS as well as previous X-ray reflectivity measurements. Additionally, we performed FCS experiments for direct determination of the molecular dynamics within the liquid film. From the fluctuation autocorrelation curve, we measured the translational diffusion of the probe molecule embedded within the fluid film formed on a solid substrate. The autocorrelation function was best fitted with two components, which indicate that the dynamics are heterogeneous in nature. However, the heterogeneity is not as pronounced as had been previously observed for molecularly thin liquid films sandwiched between two solid substrates.     

Computer simulation studies of Newton black films

We study via molecular dynamics simulations thin films (Newton black films, NBF) consisting of water coated with sodium dodecyl sulfate (SDS) surfactants. We analyze in detail the film properties (distribution of particles, pair correlation functions, roughness of the film, tilt angle of the hydrocarbon chain, electron density profiles, and mobility of water molecules) as a function of water content in the film core (i.e., film thickness, H). Our simulations indicate that water is part of the bilayer structure as solvation water. We estimate that around 2.25 water molecules per surfactant are part of this solvation structure. The structural analysis of the NBF shows that the headgroups exhibit a high degree of in-plane ordering. We find evidence for the existence of cavities in the monolayer, where only water is present. The basic structure of the monolayer is conserved down to water contents of the order of 4 water molecules per surfactant (H approximately equal to 11 A). The computed monolayer roughness for the present model is 2.5 A, in good agreement with the experimental data. We find that the roughness is very sensitive to the details of the interatomic potentials. Water mobility calculations emphasize the sluggish dynamics of very thin NBF. Diffusion coefficients of water in the lateral direction strongly decrease with film thickness. We find that the typical mean squared displacement of water in the direction normal to the bilayer is between 9 and 80 A2. Overall, our results indicate that the equilibrium SDS Newton black films studied in the X-ray experiments contain from 2 to 4 water molecules per surfactant.     

Preparation and characterization of polyaniline films in the presence of N-phenyl-1,4-phenylenediamine

The chemical oxidation of aniline to form polyaniline (PANI) films was made in the presence of N-phenyl-1,4-phenylenediamine (PPDA) in aqueous hydrochloric acid medium. The PANI films were monitored by using the quartz crystal microbalance (QCM) technique. The effect of PPDA and its concentration on the film formation was investigated. It was found that PPDA decreases the yield of the PANI film, the induction period and the depletion time of the polymerization. However, the growth rate of the film formation was found to increase by increasing PPDA concentration. These results were justified by measuring the UV-VIS absorption spectra for the in situ PANI films and the in situ UV-VIS absorption spectra for the polymer in the bulk during the polymerization. The conductivity for the PANI films at different concentrations of PPDA was measured. Also, the IR spectra, X-ray and the thermal gravimetric analysis for the PANI powder formed in the bulk in the presence of PPDA were measured and discussed.     

Adsorption behavior of hexadecyltrimethylammonium bromide (CTAB) to mica substrates as observed by atomic force microscopy

The study of the adsorption behavior of surfac-which makes people further study the adsorptiontants to interfaces is very important in colloid and in-mechanism at the molecular level.terface science[1]owing to the important applications In situ AFM measur…     

Solvation forces between silica bodies in supercritical carbon dioxide

We report Monte Carlo simulations of the solvation pressure between two planar surfaces, which represent the interface of spherical silica nanoparticles in supercritical carbon dioxide. Carbon dioxide (CO2) was modeled as an atomistic dumbbell or a spherical Lennard-Jones particle. The interaction between CO2 molecules and silica surfaces was characterized by the standard Steele potential with energetic heterogeneities representing the hydrogen bonds. The parameters for the solid-fluid interaction potentials were obtained by fitting our simulations to the experimental isotherms of CO2 sorption on mesoporous siliceous materials. We studied the dependence of the solvation force on the distance between planar silica surfaces at T = 318 K, at equilibrium bulk pressures p(bulk) ranging from 69 to 200 atm. At 69 atm, we observed a long-range attraction between the two surfaces, and it vanished when the pressure was increased to 102 and then 200 atm. The results obtained with different fluid models were consistent with each other. According to our observations, energetic heterogeneities of the surface have negligible influence on the solvation pressure. Using the Derjaguin approximation, we calculated the solvation forces between spherical silica nanoparticles in supercritical CO2 from the solvation pressures between the planar surfaces.     

Fluorescence behaviour of cyanobiphenyl liquid crystal molecules in liquid crystal/polymer composite films

The fluorescence behaviour of the liquid crystal, 4-cyano-4-pentylbiphenyl (5CB), in composite thin films prepared by the photopolymerization of 5CB/diacrylate mixtures, was investigated by means of three different excitation methods, in which the total-internal-reflection or surface-limited excitation method was used for analysis of the fluorescence from an ultra-thin interface layer ( 100nm) in contact with the substrate surface, whereas the fluorescence from the interior bulk was analysed by the through-film excitation method. It was found that intensity ratios of the monomer and excimer emissions of 5CB are significantly lower in the interface layer than in the interior bulk, depending upon photopolymerization conditions as well as upon the structures of the diacrylates used. Scanning electron microscopic observations and light-scattering measurements of some typical composite films showed possible relationships between morphological features and fluorescence characteristics depending upon the diacrylate structures and polymerization conditions. The different fluorescence behaviour has been discussed in terms of differences in mobility and/or aggregation degrees of 5CB molecules arising from dominant molecular interactions with the substrate and polymer surfaces.     

Zur theorie der α-ketosäuren : Kryoskopische untersuchungen der assoziation aliphatischer α-ketosäuren und phenylglyoxylsäuren in benzol

The type of association of aliphatic α-oxo acids, RCOCOOH, in benzene depends on the alkyl substituent R. Since this effect has also been observed in p-alkyl-phenylglyoxylic acids, it cannot be explained in terms of intramolecular steric interactions but is due to the variation in the lyophilic behaviour of the dissolved molecules.     

Atomic force microcopy study of the mechanical and electrical properties of monolayer films of molecules with aromatic end groups

The effect of intermolecular pi-pi stacking on the electrical and mechanical properties of monolayer film molecules containing aromatic groups was studied using atomic force microscopy. Two types of aromatic molecules, (4-mercaptophenyl) anthrylacetylene (MPAA) and (4-mercaptophenyl)-phenylacetylene (MPPA), were used as model systems with different pi-pi stacking strength. Monolayer films of these molecules on Au(111) surfaces exhibited conductivities differing by more than 1 order of magnitude, with MPAA being the most conductive and MPPA being the least conductive. The response to compressive loads by the AFM tip was also found to be very different for both molecules. In MPAA films, distinct molecular conductivity changes are observed upon mechanical perturbation. This effect, however, was not observed on the MPPA film, where intermolecular pi-pi interactions are likely weaker.